1. Field of the Invention
This invention, in its preferred form, relates to apparatus for laser machining a work piece and more particularly to apparatus for focusing and directing a laser beam onto the work piece. More particularly, this invention relates to apparatus for aligning the laser beam accurately such that repositioning of the laser focusing lens position does not displace the focal spot of the laser on the work piece.
2. Description of the Prior Art
The precision laser machining apparatus of this invention relates generally to the manufacture of nuclear fuel bundle assemblies 10 as shown in FIG. 1 of the . drawings. As shown, the nuclear fuel bundle assembly 10 is a self-contained unit comprised of a top nozzle assembly 12 and a bottom nozzle assembly 14, between which is disposed a matrix of nuclear fuel rods 18 arrayed in rows and columns and held in such configuration by a plurality of fuel rod grids 16. Though not shown in FIG. 1, control rods are included at selected positions within the array of nuclear fuel rods 18. The assemblies 12 and 14 and the fuel rod grids 16 provide a skeletal frame to support the fuel rods 18 and the control rods. The nuclear fuel bundle assemblies 10 are loaded into predetermined locations within a nuclear reactor and, therefore, the orientation of the fuel rods 18 with respect to each other is rigorously controlled.
The precision laser welding apparatus of this invention is, in one illustrative embodiment thereof, related to the manufacture of fuel rod grids 16 as shown in FIGS. 2A to 2C. The fuel rod grid 16 is of an approximately square configuration, whose periphery is formed by four outer grid straps 22. Each end of an outer grid strap 22 is welded by a corner seam weld 30 to the end of a perpendicularly disposed outer grip strap 22. A plurality of inner grid straps 20 is disposed in rows and columns perpendicular to each other, whereby a plurality of cells are formed to receive the control rods and the nuclear fuel rods 18. The inner grid straps 20 disposed along the rows and columns have complementary slots therein at each of the points 24 of intersection for receiving a perpendicularly disposed inner grid strap 20. An intersect weld 32 is formed at each of the points 24 of intersection, whereby a rigid egg crate structure is formed. Further, each of the inner grid straps 20 includes at each end a pair of tabs 26 of a size and configuration to be tightly received in either a top or bottom row of slots 28 formed in the outer grid straps 22, as shown in FIG. 2A. A slot and tab weld 34 is effected along the top and bottom rows formed by the slots 28 within the outer grid straps 22.
Further, a plurality of guide sleeves 36 is disposed on the sleeve side surface of the fuel rod grid 16 to receive and guide the control rods disposed therein. A series of notch seam welds 40 securely attaches the guide sleeves 36 to corresponding notches 38 formed within the inner grid straps 20. The precision laser welding apparatus of this invention is particularly adapted to perform a series of controlled welding operations whereby each of the welds 30, 32, 34, and 40 is carried out. It is understood that after each such weld, the fuel rod grid 16 is repositioned and/or the focal point of the laser beam changed to effect the particular type of weld desired.
Referring now to FIGS. 2B and 2C, the plurality of resilient fingers 44 is disposed longitudinally of the inner grid straps 20 in a parallel relationship to each other. A pair of spacing fingers 46 is disposed on either side of a corresponding resilient finger 44 and serves along with the resilient finger 44 to provide a resilient grip of the nuclear fuel rods 18 that are disposed within the cell formed by the intersecting inner grid straps 20. A resilient finger 44a is disposed to the right as seen in FIG. 2C in an opposing relationship to the spacing finger 46a, whereby a nuclear fuel rod 18 is resiliently held therebetween.
The fuel rod grid 16 is machined and in particular welded. In order to perform the intersect welds 32, the fuel rod grid 16 is incrementally moved along each of its X and Y axes, stopping at each of a plurality of positions wherein the laser beam is aligned with each of the intersections of the inner grid straps 20. Once positioned, a laser source is energized to emit a laser beam onto the aligned point of intersection to thereby effect an intersect weld 32. Thereafter, the fuel rod grid 16 is moved to the next position and another intersect weld 32 is made. The slot and tab welds 34, as well as the corner seam welds 30, are made by rotating the fuel rod grid 16 about its Y axis so that each of its outer grid straps 22 is presented to the laser beam for welding. In addition, notch seam welds 40 securing the guide sleeves 36 within the notches 38 of the inner grid straps 20 are carried out by rotating the fuel rod grid 16 to a position disposed at an angle of 45.degree. with respect to the laser beam to thereby expose the interface between the guide sleeves 36 and the slots 38 to the laser beam. The laser beam is initially focused to perform the intersect welds 32 as are carried out within a single plane in which the intersect welds lie. In order to make the corner seam welds 30 and the slot and tab welds 34, it is necessary to rotate the fuel rod grid 16 out of the plane of the intersect welds 32, thus requiring the refocusing of the laser beam. In similar fashion, the fuel rod grid 16 is rotated from the plane of the intersect welds 32 to its 45.degree. angle position with respect to the laser beam, thus also requiring a refocusing of the laser beam before precision welding may be carried out.
As described in the copending application entitled "LASER MACHINING SYSTEM" (Westinghouse Case No. 49,723); U.S. patent application Ser. No. 414,241, the refocusing of the laser beam is effected by a lens mounted within a laser focusing lens assembly that is rectilinearly moved in a controlled fashion such that the laser beam is refocused onto different focal planes corresponding to the elevation of each of the intersect welds 32, the corner seam welds 30, and the slot and tab welds 34. The laser beam passes through a beam expander, then reflected by a series of mirrors before being focused by the focusing lens 202, as illustrated in FIG. 3, onto the work piece, which may take the form of the fuel rod grid 16, as described above. As shown in FIG. 3, the focusing lens 202 may assume at least three different positions to effect the different welds, as described above. The axial displacement of the lens 202 along its axis Z-Z', which is the vertical axis of the lens assembly supporting the lens 202, causes the focal spot of the laser beam to be displaced laterally from its proper location resulting in inferior weld quality.